Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2

HIGH POWER CONTINUOUS WAVE TEST OF RF COUPLERS FOR THE
RFQ OF THE LINEAR IFMIF PROTOTYPE ACCELERATORDe Franco A.†, Hirosawa K., Ishimura K., Kaneko N., Kondo K., Kubo N., Masuda K., NakayamaT., Narita T., Sugimoto M., Yanagimachi T., National Institutes for Quantum Science andTechnology (QST), Rokkasho, Japan
Caballero C., Gonzalez-Gallego S.C. L., Maindive L., Morales J.C., University of Granada,
Granada, Spain
Garcia J. M., The Centre for Energy, Environmental and Technological Research (CIEMAT),
Madrid, SpainGex D., Moya I., Scantamburlo F.1, Fusion for Energy (F4E)
1
also at National Institute for Nuclear Physics (INFN), Legnaro, Italy
Abstract
The International Fusion Materials Irradiation Facility(IFMIF) is an accelerator-based D+ on Li target neutron
source designed to test materials for application in future
fusion reactors. The Linear IFMIF Prototype Accelerator(LIPAc) is designed to validate key components for IFMIF
accelerator. LIPAc is designed to accelerate 125 mA D+
beams to 9 MeV in CW with an ECR source (100 keV), a
RFQ (5 MeV), a yet to be installed SRF LINAC, matching
sections, and a beam dump. The RFQ requires 175 MHz
RF input of up to 160 kW in CW from each of its 8 ports.
During the RF conditioning of the cavity at high duty cycle
the RF coupler’s elastomers and alumina vacuum windows
experienced damages. A second set of RF couplers has
been designed and tested at low power with a high Q load
circuit in 2016. In this work we report the recent highpower tests (200kW CW) of two couplers with a bridge
cavity. We describe the setup, report results, and the challenges encountered including severe multipacting in thebridge cavity. To conclude we present an outlook of future
tests.

INTRODUCTION
The fusion energy community recognized the necessityof neutron irradiation facilities to test materials for use in
next generation reactors. The International Fusion Materials Irradiation Facility (IFMIF) will provide the required
neutron energy and fluence by acceleration of D+ at 40
MeV onto a liquid lithium target. Under the Broader Approach (BA) agreement, Japan and Europe designed andbuilt the Linear IFMIF Prototype Accelerator (LIPAc) in
Rokkasho, Aomori, Japan to validate the main key technologies necessary to fulfil the challenging requirements ofbeam acceleration for IFMIF. LIPAc consist of an Electron
Cyclotron Resonance (ECR) ion source capable to extract~160mA of D+ at 100 keV in CW, a LEBT, the longest
RFQ in the world to bunch and accelerate particles in CW
to 5 MeV, a MEBT that includes 2 buncher cavities, 8 Half
Wave Resonator (HWR) cavities Superconductive RF(SRF) LINAC to boost beam to 9 MeV, a HEBT, and adump capable to receive the ~ 1.1 MW beam power in CW.

The installation and commissioning of LIPAc is progressing in stages [1]. In 2024-25 the SRF will be installed, until
then a transport line replaces it to conduct systems validation and beam characterization at 5 MeV up to CW. During
the RF conditioning process of the RFQ, in December 2021,
RF power was successfully injected in CW to sustain 80%of the cavity vane voltage [2]. In early 2022, the Viton Oring sealing the vacuum RF window of two of the eight RF
couplers of the RFQ got damaged and partially melted [3].
The original design of the RFQ RF couplers foresaw a ceramic window brazed to the metal body [4]. Tests of a prototype revealed ~30% insertion losses, which were explained by a too thick TiN coating of 100 nm on the alumina window to suppress secondary electron emissions.
This set of couplers was redesigned and manufactured in2016 with an appropriate ~5 nm TiN coating. Due to a lackof availability of high-power RF equipment, they were
tested with a high load I-Q circuit to be validated undersimilar EM fields expected while connected to the RFQ [5].
In the meantime, a backup set of couplers with vacuum
sealed by Viton O-rings were designed, manufactured, andtested at high power in CW to minimize risk of impact toproject schedule [6]. Due to their excellent performance on
the test bench, they were installed in the RFQ and used forall operation of LIPAc so far. Since 2019 it became evident
that five of the eight couplers exhibit external body temperature significantly higher than others, which we hypothesis are due in part to multipacting (MP) [7] and in part toinsufficient heat extraction from the inner conductor near
the ceramic window. In 2022-23 the design of these couplers was significantly improved, and an upgraded version
was installed to continue beam operations towards high d.c.
[8]. In parallel, the original couplers were tested to validate
thermomechanical and RF properties at high-power in CW.

TEST BENCH
The nominal RFQ operation requires RF injection at 175
MHz of ~600 kW during conditioning and ~1.2MW considering beam loading during acceleration. The RF is provided by 8 chains each capable to provide up to 220 kW in
CW and synchronized by a LLRF powered by White Rabbit [9]. To complete the validation of the original RF cou-___________________________________________

defranco.andrea@qst.go.jp†- 34 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2

plers, it is necessary to demonstrate that their thermomechanical properties are adequate to operate with >200 kW
forward power (FWD) and 160 kW FWD + at least 20 kW
reflected power (REV) in CW. To perform this test the set
up shown in Fig. 1 was developed. One of the RF chains
designed for RFQ operation injects up to 220 kW througha coupler (CPL1) to a re-entrant resonant cavity thatbridges the RF to a second coupler (CPL2) towards acooled and matched load. To provide efficient cooling to
the inner conductor of the couplers, they are by design
paired with a λ/4 T shaped waveguide (T-WG).

Figure 2: Thermal simulation of bridge cavity, tuner andcouplers not shown. Top left: internal surface; Top right:
external lid surface; Bottom: cutout view.

VacuumFigure 1: Schematic view of high power test bench for LIPAc RFQ`s RF couplers.

Cooling
The cavity is manufactured in aluminium, because it islightweight, is significantly cheaper than copper, has goodthermal conductivity and decent electrical conductivity.
The cavity insertion losses are measured with virtual network analyser to be ~12%, which translate to ~26 kW ofheat to be extracted from the inner surfaces of the cavity. A
spiral channel is machined in the central stub to provide~16 L/min of cooling water. Aluminium tends to easily corrode in the presence of many other metals and in particularcopper, so a fine mesh filter (50 µm) was installed at theoutlet to trap debris, which presence was confirmed bychemical analysis of filter after completion of test. Additionally, the walls of the cavity are excavated to host threecopper serpentine pipes to circulate additional ~40 L/min
in total of cooling water. Figure 2 shows the expected temperature distribution of the different parts of the cavity,
which are expected to increase the outgassing of surfaces,
but still be acceptable. Each coupler has three water cooling lines, each with a flow of roughly ~5 L/min: a spiralwithin the antenna and nearby portion of outer and innerconductor, a circle in the external body around the alumina
window, and the inside of the inner conductor from near
the antenna up to the λ/4 protrusion of the T-WG. The load,
circulator, and amplifiers of the RF chain are separately
cooled by their skid.

A ~80 L/s turbomolecular pump is connected to a DN 63
CF flange which is covered with a copper grid with circle
holes covering ~60% area to minimize current disruption
inside the cavity. A scroll pump follows with a line forpurging with nitrogen. A large Viton O-ring seals the lid of
the cavity to its main body, with E-serts used to facilitate
bolting. SUS flanges with thick Cu layer for couplers andthe insertion of a tuner are also sealed with Viton O-rings.
The cavity and couplers were baked at ~100 ºC for ~72
hours before operation. RF fingers are placed in all the gaps
were current flows. The typical vacuum pressure during
operation is around 5∙10-7 mBar.

Instrumentation
The test bench is equipped with a series of sensors tomonitor the experiment and interlock RF injection whenever necessary. A cold cathode gauge measures the vacuum
pressure inside one of the two couplers, a Residual Gas Analyser (RGA) measures the vacuum partial pressure ofcomponents with total atomic mass lower than 100. To perform calorimetry measurements, the cooling water flow ismeasured in every channel and the inlet/outlet water temperature is sensed with PT100 installed on the surface of
the SUS fitting of central cavity stub cooling circuit and theones of CPL1 near the antenna and alumina window. A
light sensor is installed on each of the coupler, looking toward the inner conductor near the alumina window to detect possible arcs or MP in vacuum. Arc sensors are also
installed in the elbows of coaxial lines outside vacuum andinside each port of the circulator. Two InfraRed (IR) cameras monitor the surface temperature of the entire test
bench from different angles. Several directional coupler
antennas are connected to an oscilloscope to monitor theflow of RF power, tough in this work we will mostly focus
on the forward power from circulator to CPL1 (FWD), thereflected power from CPL1 back to circulator (REV) and- 35 -

Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2

the power transmitted to CPL2 and forwarded to the load
(TRA) as shown in Fig. 1. The maximum expected electricfield in the cavity`s capacitive gap of 20 mm is ~9 kV/mm,
so the entire test bench is surrounded with a double layerof 1 mm thick lead sheet to shield personnel and equipmentby X-ray emissions. A proportional counter measures thedose outside the shielding for safety reasons. No emission
above background level was detect throughout the entire
test. Three PT100 are installed on the cavity and each of
the coupler external surface to monitor where simulations
predict the highest temperatures.

Control
The control and most of the data archiving (excluding
RGA, IR, oscilloscope, flowmeters) is implemented in EPICS. By design, the cavity resonant frequency is very sensitive to the width of the capacitive gap between central
stub and the lid (~3 MHz/mm). A ~6 cm diameter copper
circle in the middle of the lid can be inserted/extracted bya manual linear motion drive to tune the cavity resonant
frequency. To facilitate its operation, we developed and installed an actuator based on a step electric motor that canbe controlled remotely by a Raspberry Pi microcontroller.

TEST
As the cavity and couplers surfaces required RF conditioning, the ramp up of FWD to CW at 220 kW was envisaged in three phases.

Phase 1
Pulse repetition period (RP) is kept at 1s and pulse width(PW) at 10 µs. The loaded Q of the cavity is ~300, the injected power is increased/decreased linearly to the plateau
value in 7 µs to minimize spikes of REV at the beginning/end of the pulse. FWD plateau power is slowly
ramped to 220 kW. This stage is completed within roughly
1 day of operation without detection or arcs, but roughly
one pulse in 100 would be nearly fully reflected.

Phase 3
While keeping FWD at 220 kW and RP of 10 ms, we
slowly elongate PW. After stretching the pulses to a few
dozens of µs, it became evident that the REV shape was
not constant within the pulse. The cavity tuner position wasscanned to minimize REV, but the result was only a shift intime of its waist within the pulse as shown in Fig. 3. It isalso at this stage that we notice that the vacuum main component is water after roughly one hour without power injection. As soon as pulses of more than few kW are injected,
the water content decrease rapidly, while oxygen and hydrogen increase at same rate until becoming the dominant
residual gases. MP and/or plasma formation in the cavity
could explain both behaviours. We first test whether MP
can be suppressed with progressive RF conditioning of surfaces. After >12 hours long operation at constant setting wemeasure no statistically significant difference of vacuum
pressure, its component, RF power balance, pulse shape orcomponent surface temperature. MP is a resonant phenomenon that often occurs within a band of power/frequency,
so we measure the pulse shape over a large range of combinations of FWD power and frequency provide by RF
chain. Figure 4 shows that for FWD above few kW, there
is no suitable operation point where REV is flat within
pulse and is an acceptable percentage of FWD. Figure 5
shows the Pareto front of REV flatness and average for
FWD > 50 kW with 3 examples. We conclude that
MP/plasma formation exists for FWD higher than few kW
in the entire frequency band that the RF amplification chain
is designed for.

Phase 2
While keeping FWD at 220 kW, the PW is increased
slowly to 15 µs and then the RP is decreased from 1 s to 10
ms within roughly 72 hours. Although no arc was detected,
every ~5 minutes, the injected power would suddenly betotally reflected starting the middle of the pulse leading us
to suspect either a poor sensitivity of the arc detectors, orthe location of the arc to be inside the cavity, far from thelight sensors near the windows of the couplers. During thisphase the vacuum pressure during the test improves from~1∙10-6 mBar to ~6∙10-7 mBar. In this phase we notice that
the REV shape flickers pulse to pulse during the linear
ramp up of the FWD. We hypothesise that a MP region is
encountered at intermediate power levels, so we significantly speed up the first half of the ramp up/down. As expected, this leads to higher instantaneous REV during loading/unloading of the cavity, but at the same time the pulse
shape is significantly more stable and frequent micro outgassing sensed by vacuum gauge disappear.

Figure 3: REV pulse shape for FWD = 220 kW, PW = 1
ms, RP = 20 ms; From top to bottom the cavity tuner islinearly swept to change cavity resonant frequency by a total of ~30 kHz.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2

Figure 4: Scan of FWD power and its frequency (expressedin reference to 175MHz). Top: ratio of average REV and
FWD within the pulse; Bottom: Flatness of REV within thepulse expressed as the difference between its maximum
and minimum normalized to the average (0 means completely flat).

High Duty Cycle
Although it is not possible to avoid MP, we try to validate
the thermomechanical design of the couplers by subjecting
CPL1 to high duty cycle and power injection with a very
substantial REV component, which constitute even harsher
condition that necessary to complete the test. Figure 6
shows the pulse shape for a duty cycle of ~96% (PW = 3.85
ms, RP = 4 ms) with FWD at ~175 kW with REV oscillating within the pulse at an average of ~115 kW. The TRA
oscillate around ~30 kW, clearly not enough to make any
conclusion on the performance of CPL2. The duty cycle is
not further increased as the circulator struggles to avoid
peaks of reflected power towards the final tetrode amplifier
due to fast phase shifts by MP. Figure 7 shows the photo of
the test stand in visible light and IR at this operation point.
The maximum temperature on CPL1 is 28.5 °C which isonly ~3 °C higher than cooling water. We estimate fromcooling water calorimetry measurements that ~75 W are
dissipated near the coupler window, ~600 W near the antenna loop. Figure 8 shows the spatial distribution of X-ray
dose integrated over roughly 48 hours of operation at a duty
cycle of 2%. It is likely that the major X-ray contribution
is located at the bottom gap of the cavity and the two sensors located at the centre are partially shielded by the thickcentral stub.

Figure 5: Pareto front for FWD > 50 kW of the compromisefor flatness of REV and its average within the pulse. Three
examples of pulse shape and their position on the Pareto
front.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2

Figure 6: Pulse shape at 96% d.c., PW = 3.8 ms, RP = 4 ms.

test. Top left shows traces of MP/arcs near the tuner, with
RF fingers that protruded and melted due to heat. Top rightshows heat marks from RF losses on the cut for air pocket
evacuation from near the cavity lid O-ring. Bottom rightshows the deformation by heat of the copper layer of CPL1
input flange likely due to poor initial bonding. Black heatmarks on CPL1 near this part are visible. For future tests
we plan to plate the entire cavity surface and couplers input
flanges with copper to reduce losses, secondary electron
emissions and avoid introducing gaps between metals. A
Cu mesh will be added on evacuations cuts and the tuner
will be removed to avoid edges and gaps in the high electricfield region between the central stub and the lid. RF andthermomechanical simulations are ongoing to verify if cavity detuning due to deformations at CW can be accommodated by the limited bandwidth of the RF amplifier chain.

Figure 9: Visual inspection of cavity after test - see text.
Figure 7: Photo from two different angles of the test bench
whit FWD ~ 175 kW, duty cycle of 96%. Left: visible light;
Right: infrared.

CONCLUSION
Two RF couplers designed for LIPAc`s RFQ have beentestes with high power at nearly CW. The test could not be
performed as designed due to persistent MP/plasma formation in the bridge cavity used. Nonetheless one of thecouplers was subjected to much harsher condition than test
goal and exhibited excellent thermomechanical behaviour
with negligible surface temperature increase and more thanadequate cooling. The bridge cavity will be repaired andimproved to supress MP and all the other 8 couplers produced will be tested to complete their validation and RF
conditioning for potential use in LIPAc RFQ in the following years.

REFERENCES

Figure 8: Dose integrated over 48 hrs at 2 % d.c. with FWD~200 kW.

Cavity Visual Inspection
After the completion of the test the cavity was disassembled for a visual inspection. Figure 9 shows the main discoveries during visual inspection inside of the cavity after[1] Dzitko, H. et al., “Status and future developments of the Linear IFMIF Prototype Accelerator (LIPAc).”, Fus. Eng. Des.,
168, 2021, 112621.
[2] Masuda et al., “Commissioning of IFMIF Prototype Accelerator towards CW operation.”, in Proc. of LINAC2022, Liverpool, UK, 2022.
[3] De Franco et al., “RF conditioning towards continuous waveof the RFQ of the Linear IFMIF Prototype Accelerator.”, in
Proc. of IPAC 23, Venice, Italy, 2023.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 WEOB2[4] S. Maebara et al., “Development of RF input coupler for the
IFMIF/EVEDA prototype RFQ LINAC.”, in Proc. of the 10th
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[5] S. Maebara et al., “High-power tests of RF input coupler for
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[6] E. Fagotti et al., “The couplers for the IFMIF-EVEDA RFQ
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[7] Masuda et al., “Present Status of Linear IFMIF Prototype Accelerator (2) Particle Simulations of Multipactering in RF
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[8] F. Scantamburlo et al., “Linear IFMIF Prototype Accelerator(LIPAc) Radio Frequency Quadrupole’s (RFQ) RF couplers
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[9] M. Weber et al., "Functional Overview of the RF Power System for the LIPAc RFQ.", in IEEE Transactions on Plasma
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